Supplements to give kitten to help her put on some weight

Tygralily, our 8 week old kitten, is a little smaller and skinnier looking than her twin sister Sashi.  Sashi is much more filled out, while Ty kind of looks a little unhealthy.  We recently took them to get spayed and the difference in weight was just an ounce and a half, but with them being as small as they are, it is noticeable visually.  They both are eating well and we recently put out separate bowls to ensure that there was no competition when it comes to eating, although they usually eat at different times and I see them both eating alone multiple times daily.

I wanted to see if anyone has any recommendations for any types of supplements I can give Ty, maybe some kind of high cal stuff that's similar to NutriCal that you give puppies? 

Isn't Nutrical good for cats (kittens) too?  I thought it was, but could be mistaken.  Did you ask your Vet is there was anything specific you could do, other than just feeding her more?  What are you feeding them?  Does she finish each meal?

mrsgreenjeens said:

Isn't Nutrical good for cats (kittens) too?  I thought it was, but could be mistaken.  Did you ask your Vet is there was anything specific you could do, other than just feeding her more?  What are you feeding them?  Does she finish each meal?

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The vet just told me free feed them and make sure they are eating every few hours, which they do.  I watch them.  

The kind of Nutrical I have isn't the actual Nutrical, it's a GNC pet version and mine says just for puppies on it; it says Puppies Stage 1.  They eat Royal Canin babycat 34, weaning to four months.  I am thinking of switching them to Wellness Kitten at four months, maybe sooner.  

The first ingredient in the Royal Canin is chicken meal and chicken fat....Wellness always has an actual meat as the first ingredient.  I mean, they love it.  But I'd rather them have better quality dry food.